Sheared electrical steel sheet



Patented July 30, 1940 UNITED STATES PATENT OFFICE SHEARED ELECTRICALSTEEL SHEET of West Virginia Application July 25, 1938, Serial No.221,172

9 Claims.

The present invention relates to iron alloys for use as magnetizableparts of electrical machinery, for example: laminated cores fortransformers, motors, and dynamos. Iron alloys for this use arefrequently called electrical steels,

and for the sake of brevitythey will be so designated herein.

The suitability of an electrical steel for a given purpose dependsprimarily on its magnetic and electrical properties, its physicalproperties, and its cost in the fully fabricated condition. Thesefactors depend upon the composition, heat treatment and mechanicaltreatment of the steel in question, and are usually interrelated to aconsiderable extent. In general, it is desired to secure a materialhaving, under the conditions of use, the highest possible permeabilityand lowest possible watt loss characteristics consistent withsatisfactory fabricating properties, adequate mechanical strength, andappropriate cost in the fabricated form.

The main bulk of electrical steel produced at the present time consistsof plain carbon steel and the so-called low, medium, and high siliconsteels. Although the magnetic properties of these steels steadilyimprove as the silicon content is increased, the steels becomeincreasingly brittle and large grained, thereby becoming more difiicultto roll and to shear with a smooth edge 30 free from cracks extendinginto the sheet. The practical upper limit of silicon content has beenabout 5.5%. With such material ithas been of utmost importance to keepthe percentages of impurities, especially carbon, ,extremely small: atlow total carbon contents an increase in the carbon content amounting toonly a few thousandths of one per cent. appreciably increases hysteresisloss and decreases the maximum permeability of the alloy.

Objects of this invention are to provide electrical steel sheet having apercentage of silicon substantially higher than 5.5%, rolled and shearedwith a smooth edge substantially free from cracks extending into thebody of the steel';

to provide a high silicon electrical steel having steel having bettermagnetic properties than other silicon electrical steels containingsubstantially more, or substantially less, silicon than the steel of theinvention; to provide a high silicon electrical steel containingconsiderably 5 higher percentages of carbon than have been usedcommercially in the 4.5% to 5.5% silicon electrical steel heretoforeused; and to provide means for broadening the range of silicon contentthat produces optimum magnetic properties 0 in electrical sheet.

There is an optimum silicon content for high silicon electrical steel.This optimum is above 5.5% silicon and at the optimum silicon contentthe magnetic and electrical properties of the material are better thanthose of materials, otherwise similar, containing either substantiallymore or substantially less silicon than the optimum.

' The invention is based in part on my discovery that the ductility,grain size, and general mechanical and shearing properties of the steelcontaining the optimum silicon content may be improved by theaddition'of substantial amounts of carbon and the use of suitable heattreatment, to such an extent that the steel so treated is, in respect ofits mechanical and magnetic properties, at least as suitable forcommercial use as the low carbon 4.5% to 5.5% silicon electrical steelsheet heretofore used. Further, I have found that if a proper heattreatment is used, the carbon added to improve the mechanical propertiesof the steel in question does not destroy the superior magneticproperties of the steel. I have also found that the addition of moderateamounts of certain elements, notablyv manganese and aluminum, broadensthe range of silicon percentages which yield the optimum magneticproperties and improves the ductility of the steel.

The invention is embodied in punched or otherwis'e sheared electricalsteel sheet comprising as essential constituents, aside from'the iron,5.5% to 7% silicon and 0.2% to 0.9% carbon, and in suitable processesfor its production and heat treatment. The preferred range of silicon 45is between 6% and 7%, and the carbon content is preferably below 0.5%.Optional constituents which favorably modify the mechanical or magneticcharacteristics, or both, of the steel are the austenite-forming metalsof the group man- 50 ganese, nickel, copper, cobalt, and silver inamounts between 0.3% and 2% each, although the preferred range of silveris 0.05% to 0.15%; the deoxidizing elements of the group aluminum,calcium, zirconium, beryllium, and boron, in amounts not exceeding about1% each; the carbide forming elements of the group chromium, molybdenum,tungsten, titanium, columbium, and tantalum in small amounts notexceeding a total of about 0.25%, although chromium may be as high as1%; and small amounts of one or more elements of the group arsenic,phosphorus, tin, and antimony. The preferred compositions fall withinthe limits specified in table A.

TableA Per cent. Silicon 6.3 to 6.7 Carbon 0.3 to 0.4 Manganese 0.6 to0.9 Copper to 0.75 Aluminum 0 to 1.5 Iron The remainder By the use ofsuitable heat treatment, as described more fully hereinafter, the steelof the invention may be forged and rolled to the 29 gage (0.014 inchthickness) sheet ordinarily used for electrical apparatus, and even tothinner sheet if desired; this sheet may be cold punched or otherwisesheared readily with a good edge; and the resulting electrical sheet maybe given excellent magnetic properties.

The improvement in magnetic and other electrical characteristicsprovided by the invention is indicated by the data appearing in Table B.These data show the relative improvement rather than the best propertiesattainable under commercial conditions.

the dimensions 0.5 inch by 0.5 inch by 10 inches. The accuracy of thepermeameter and of the test procedure was insured by checking several ofthe experimental results with comparison tests made by the U. S. Bureauof Standards. The values of hysteresis represent the areas of hysteresisloops at a maximum induction of 10 kilogausses, and are expressed inergs per cubic centimeter at 10 kilogausses.

The data under the general heading Direct current measurements"indicate, first, that the magnetic properties of low carbon siliconsteels reach an optimum in the neighborhood of 6.5% silicon. Second,raising the carbon to 0.35% or 0.45% does not greatly change the optimumsilicon percentage, does deleteriously affect the magnetic properties ofsteels having substantially less or more silicon than the optimum, butdoes not deleteriously affect, and may even improve, the magneticproperties of steel containing about 6.5% silicon. Third, the additionof manganese, 0.75% to 0.8% for instance, broadens the range of siliconpercentages within which substantially optimum magnetic properties canbe attained in the high carbon steels.

In the same Table B, the data under the heading Losses at 60 cycles wereobtained by standard core loss tests, using alternating current andstandard Epstein strip samples (0.014 inch thick) prepared as specifiedby the American Society for Testing Materials. The hysteresis and eddycurrent losses were separated by the well-known two-frequency method,using 60 cycle and 30 cycle currents, respectively. These data indicatethat the total core losses at 60 cycles and 10 kilogausses are about 25%lower in the high carbon 6.4% silicon material than in the low carbon4.5% silicon steel.

Table B Composition (remainder Fe) D. 0. measurements Losses at 60cycles No. Percent Percent Percent Hyster- Resistg a rmees curren 0 Mnem ability watts/lb. watts/lb.

0. 03 0.07 4. 54 2. 269 4, 600 54. 7 0. 03 Low 6. 39 1, 298 12, 400 70.8 0. 03 Low 6. 61 1, 554 9, 500 74. 0 0. 06 Low 7. 45 2, 268 5, 100 77.3

0. 41 Low 4. 80 5, 715 2, 300 49. 3 0. 36 0. 12 6. 37 1, 044 15, 600 72.7 0.42 Low 6.66 1,988 6 550 75.2 0. 44 Low 7. 53 3, 568 50 80.8

'Microhms per cubic centimeter.

All samples, except No. 5, on which direct current measurements weremade, were annealed at 900 C. in hydrogen for 6 hours and cooled in thefurnace; sample 5 was annealed at 820 C. in

hydrogen for 6 hours and furnace cooled. The samples on whichalternating current measurements were made, except for sample No. 1,were annealed at 1050 C. in hydrogen for 6 hours, slowly cooled to 900C. and there held for 6 hours, slowly cooled to 875 C. and there heldfor 6 hours, and furnace cooled; sample No. 1 was annealed at 900 C. inhydrogen for hours and furnace cooled.

The data appearing under the headings Hysteresis and Maximumpermeability," in Table B, were obtained by the use of a Fahy Simplexpermeameter and magnetic test specimens with The relatively low eddycurrent losses characteristic of the high-carbon, high silicon steel areprobably the result of the high specific resistance of the material.

The physical characteristics of the steel of the invention are indicatedby the data in Table C. These data were obtained by hot rolling siliconsteel to sheet 0.014 inch thick, heat treating the sheet to toughen it,cold shearing the sheet into samples one-half inch by two inches,bending the samples, transverse to their longer axes, about athree-sixteenths inch diameter round pin with a bending radius ofthree-fourths of an inch, until incipient cracking began at the bends,and

measuring the angle of bend required to start a best, only a roughapproximation of relative ductilities, I have found from actual shearingtests that the relative ductilities in Table are in the same order asthe relative suitability of the steelsfor shearing.

Table C Composition (remain Steel as'rolled Steel ann l d' der iron) eae Perma- Total Perma- Total 8 h fi Pew-em nent set, bend, nent set,bend,

degrees degrees degrees degrees 0. Low 4. 77 180 180 46 80 0. 03 Low 5.53 20 54 35 0. 03 Low 6. 37 0 19 0 17 0. 06 Low 7. 45 0 5 0 12 0. 41 Low4. 80 123 176 180 180 0. 38 Low 5. 82 53 87 162 180 0. 42 Low 6. 66 2 287 34 0. 44 Low 7. 53 0 12 0 0. 27 0. 75 6. 25 14 35 75 104 0. 35 0. 756. 40 28 51 110 133 0. 44 0. 78 6. 23 36 61 50 87 As described hereinbelow.

As indicated by the data in this Table C, the ductility and toughness oflow carbon steel decrease rapidly as the silicon is raised above 5%, andbecome negligible above 6% silicon. Annealing these low carbon steelsserves only to decrease the ductility still further, because of thecoarsening of the grain size brought about by such heat treatment. Theaddition of carbon not only raises the ductility and toughness of thesteel in the as-rolled state, but also makes the steel amenable to atoughening anneal. In the absence of manganese, the addition of about0.35% carbon and the use of a toughening anneal renders a 6.5% siliconsteel about astough and ductile as a low carbon 5.5% silicon steel. Thefurther addition of about 0.75% manganese raises still further theductility and toughness of the high silicon steel.

A clearer understanding of the improvements in the magnetic propertiesof silicon steels brought about by the present invention may be attainedby referring to the accompanying drawings, in which Figures 1 to 5,inclusive, are the normal magnetization curves (dotted lines), and thenormal hysteresis half-loops ,(solid lines) plotted'at a maximummagnetization of ten kilogausses, of five silicon electrical steels;

Figure 6 is a graph indicating the effect of various silicon contents,and of manganese, upon the maximum permeability .of medium carbon steel;and

Figure 7 is a graph indicating the effect of various silicon contents,and of manganese, upon the normal hysteresis of medium carbon steel atten thousand gausses. All of the figures, 1 through 7, are based onexperimental data obtained through the use of a Fahy Simplexpermeameter. In Figures 1 through 5, the magnetizing force is expressedin Oersteds.

Referring specifically to Figures 1, 2, and 3, it will be observed thatthe low carbon 4.5% silicon steel of Figure 1 has a high permeability atmagnetizations up to about 10 kilogausses, a low residual magnetization(Br), a small coercive force (He), and low hysteresis. Increasing thecarbon, as in Figure-2, produces relatively a very great decrease inpermeability and increases in residual magnetization, coercive force,and hysteresis. When, however, both the carbon and the silicon areincreased, and a suitable heat treatment is employed, magneticproperties similar to those indicated in Figure 3 are obtained.

If the silicon content is too great, even the high carbon material hasrelatively poor magnetic properties, as shown in Figure 4.

The addition of manganese to a steel containing suitable percentages ofsilicon and carbon has a favorable effect upon the magnetic properties.This effect is illustrated in Figure 5.

Referring specifically to Figures 6 and 7, the

curves labelled A represent the behavior of high carbon (0.4% C) siliconsteels containing little or no manganese, and the curves labelled Brepresent the characteristics of high carbon (0.4% C) silicon steelscontaining about 0.75% manganese. It will be observed that the highestmaximum permeability is obtained at substantially the same siliconpercentage as the lowest hysteresis. It will be further observed thatthe general effects of manganese on the magnetic properties are tobroaden the range of optimum silicon percentages, especially at thelower silicon percentages, and to improve the magnetic propertiesattainable at the optimum silicon percentages. That is, in the B curves,the hysteresis and permeability do not change as rapidly with changes insilicon content on each side of the maximum, as they do in the A curves.Manganese also seems to shift the optimum silicon percentage (the cuspsof the curves) to a slightly higher percentage.

In considering cusp type curves such as those of Figures 6 and 7, itshould not be forgotten that, in actual practice, it is not possible toattain the properties indicated by the extreme tip of the cusp,primarily because of microsegregation in the steel; but such idealproperties can be approached fairly closely, as indicated by acomparison of Table B with Figures 6 and 7.

The effects of nickel, silver, cobalt, and copper additions to the highcarbon high silicon steel are to increase the toughness and theelectrical resistivity of the steel. The carbide-forming elements, suchas chromium, titanium, molybdenum, tungsten, vanadium, columbium, andtantalum, have little eifect on the range of optimum siliconpercentages, but have a beneficial effect on the toughness of the steel.

Deoxidizing elements, such as calcium, aluminum, zirconium, beryllium,and boron, tend to improve the hot working characteristics anduniformity of the steel of the invention, although in percentages above1% or 2% they reduce somewhat the ductillty and saturation induction.These elements broaden somewhat the range of optimum silicon percentagesand shift the range slightly in the direction of lower silicon: Aluminumis particularly effective in this respect, and if 4% aluminum is added,the hysteresis value does not rise' prohibitively with silicon as low asabout 4.5%.

The high carbon, high silicon steel of the invention may be forged androlled without difiiculty at about 1150 C. to 900 C., and the finishingtemperature of thin sheet may be somewhat below 500 C. There is someadvantage to be gained by finishing the rolling operation, even in thinsheet, at about 700 C. or somewhat higher. It is also advisable to avoidexcessivedecarburization caused by unduly prolonged holding in the airat high temperatures.

After rolling into sheet, thesteel may be annealed, and then toughenedfor shearing by rapidly cooling it to'room temperature from an elevatedtemperature. For the annealing treatment, satisfactory results have beenattained by holding the sheet at temperatures between 650 C. and 1050 C.for a time ranging from 48 hours at the lower temperatures to about oneminute at the higher temperatures, and the best results can be obtainedby holding the sheet at 700 C. to 900 C. for one-half hour to twentyhours. The toughening treatment may be effected by simple cooling in airfrom the annealing treatment or by reheating fo a short time, say 5 to15 minutes, and then air cooling. A more rapid quench than air coolingmay be employed if desired. The toughening treatment produces a finegrained structure.

Shearing and punching may be done while the sheet is cold or heated to atemperature not over several hundred degrees centigrade. After shearing,it is advisable to subject the sheet to another heat treatment todevelop the optimum magnetic properties. This latter heat treatmentconsists in heating the steel at a temperature within or above thecritical range (about 1000 C. to 1200 C.) and then cooling it to atemperature (700 C. to 900 C.) below the critical range. The coolingrate should be sufiiciently slow to insure that substantially all of thecarbon exists as discrete particles so distributed as to have noseriously deleterious efiect on the magnetic properties of the sheet. Atypical procedure is to heat the steel at 1050 C. for six hours, cool itslowly to 900 C., and furnace cool it to a black heat, whereuponsubstantially all of the carbon will be found to be in the form ofgraphite particles. It is preferred that this heat treatment, and alsothe previously described toughening anneal, be effected in an inert orreducing atmosphere, such as hydrogen or a hydrogen-nitrogen mixture.

This application is in part a continuation of my application Serial No.94,727, filed August 7, 1936.

I claim:

1. Punched or otherwise sheared electrical steel sheet having smoothedges substantially free from cracks extending into the body of thesheet, suitable for use as magnetic core material in alternating currentelectrical devices, and having substantially the composition: 5.5% to 7%silicon, 0.2% to 0.9% carbon, a small but efi'ective amount notexceeding 1% of deoxidizing material of the group consisting ofberyllium, boron, aluminum, calcium, zirconium, and mixtures thereof,and the remainder iron; practically all of the carbon being in the formof discrete particles so distributed as not to have any seriouslydetrimental efiect on the magnetic properties of the sheet.

2. Punched or otherwise sheared electrical steel sheet having smoothedges substantially free from cracks extending into the body of thesheet, suitable for use as magnetic core material in alternating currentelectrical devices, and having substantially the composition: 6% to 7%silicon, 0.2% to 0.5% carbon, 2. small but efiective amount notexceeding 1% of deoxidizing material of the group consisting ofberyllium, boron, aluminum, calcium, zirconium, and mixtures thereof,and the remainder iron; practically all of the carbon being in the formof discrete particles so distributed as not to have any seriouslydetrimental efiect on the magnetic properties of the sheet.

3. Punched or otherwise sheared electrical steel sheet having smoothedges substantially free from cracks extending into the body of thesheet, suitable for use as magnetic core material in alternating currentelectrical devices, and having substantially the composition: 6.3% to6.7% silicon, 0.3% to 0.4% carbon, a small but efiective amount notexceeding 1% of deoxidizing material of the group consisting ofberyllium, boron, aluminum, calcium, zirconium, and mixtures thereof,and the remainder iron; practically all of the carbon being in the formof discrete particles of graphite so distributed as not to have anyseriously detrimental efiect on the' magnetic properties of the sheet.

4. Punched or otherwise sheared electrical steel sheet having smoothedges substantially free from cracks extending into the body of thesheet, suitable for use as magnetic core material in alternating currentelectrical devices, and having substantially the composition: 5.5% to 7%silicon, 0.2% to 0.9% carbon, a small but efiective amount not exceeding1% of deoxidizing material of the group consisting of beryllium, boron,aluminum, calcium, zirconium, and mixtures thereof, and the remainderiron; which sheet has been toughened by being heated at a temperaturebetween 650 C. and 1050 C. and rapidly cooled to produce a fine-grainedstructure; then punched or otherwise sheared; and thereafter heattreated, to improve its magnetic properties, by cooling it from a.temperature between 1000" C. and 1200 C. at a rate sufiiciently slow toinsure that substantially all of the carbon is in the form of discreteparticles so distributed as not to have any seriously detrimental efiecton the magnetic properties of: the sheet.

5. Punched or otherwise sheared electrical steel sheet having smoothedges substantially free from cracks extending into the body of thesheet, suitable for use as magnetic core material in alternating currentelectrical devices, and having substantially the composition: 6% to 7%silicon, 0.2% to 0.5% carbon, a small but efiect-ive amount notexceeding 1% of deoxidizing material of the group consisting ofberyllium, boron, aluminum, calcium, zirconium, and mixtures thereof,and the remainder iron; which sheet has been toughened by being heatedat a temperature between 650 C. and 1050 C. and rapidly cooled toproduce a fine-grained structure; then punched or otherwise sheared; andthereafter heat treated to improve its magnetic properties, by coolingit from a temperature between 1000 C. and 1200 C. at a rate suflicientlyslow to insure that substantially all of the carbon is in the form ofdiscrete particles so distributed as not to have any seriouslydetrimental effect on the magnetic properties of the sheet.

6. Punched or otherwise sheared electrical steel sheet having smoothedges substantially free from cracks extending into the body of thesheet, suitable for use as magnetic core material in alternating currentelectrical devices, and haVing substantially the composition: 6.3% to6.7% silicon, 0.3% to 0.4% carbon, a small butefiective amount notexceeding 1% of deoxidizing material of the group consisting ofberyllium, boron, aluminum, calcium, zirconium, and mixtures thereof,and the remainder iron; which sheet has been toughened by being heatedat a temperature between 700 C. and 900 C. and rapidly cooled to producea fine-grained structure; then punched or otherwise sheared; andthereafter heat treated, to improve its magnetic properties, by coolingit from a temperature between 1000" C. and 1200 C. at a ratesufliciently slow to insure that substantially all of the carbon is inthe form of discrete particles of graphite so distributed as not to haveany seriously detrimental efiect on the magnetic properties of thesheet.

'7. Method of producing a punched or otherwise sheared electrical steelsheet having smooth edges substantially free from cracks extending intothe body of the sheet and suitable for use as magnetic core material inalternating current electrical devices, which method comprises hotrolling to sheet form a steel containing 5.5% to 7% silicon, 0.2% to0.9% carbon, 8. small but effective amount not exceeding 1% ofdeoxidizing rate sufliciently slow to insure that substantially all ofthe carbon is in the form of discrete particles so distributed as not tohave any seriously detrimental efiect on the magnetic properties of thesheet.

8. Method of producing a punched or otherwise sheared electrical steelsheet having smooth edges substantially free from cracks extending intothe body of the sheet and suitable for use as magnetic core material inalternating current electrical devices, which method comprises hotrolling to sheet form a steel containing 6%, to 7% silicon, 0.2% to 0.5%carbon, 9. small but effective amount not exceeding 1% of deoxidizingmaterial of the group consisting of beryllium, boron, aluminum, calcium,zirconium, and mixtures thereof, remainder substantially all iron;imparting good shearing properties to such rolled sheet by heating it ata temperature between 700 C. and 900 C. and rapidly cooling it toproduce a fine-grained structure; then punching or otherwise shearingthe toughened sheet; reheating the shearedsheet to a temperature between1000 C. and 1200 C., and cooling it at a rate suiilciently slow toinsurethat substantially all of the carbon is in the form of discreteparticles so distributed as not to have any seriously detrimental effecton the magnetic properties of the sheet.

9. Method of producing a punched or otherwise sheared electrical steelsheet having smooth edges substantially free from cracks extending intothe body of the sheet and suitable for use as magnetic core material inalternating current electrical devices, which method comprises hotrolling to sheet form a steel containing 6.31% to 6.7% silicon, 0.3% to0.4% carbon, a small but etfective amount not exceeding 1% ofdeoxidizing material of the group consisting of beryllium, boron,aluminum, calcium, zirconium, and mixtures thereof, remaindersubstantially all iron; imparting good shearing properties to suchrolled sheet by heating it at a temperature between 700 C. and 900 C.and rapidly cooling it to produce a fine-grained structure; thenpunching or otherwise shearing the toughened sheet; reheating thesheared sheet to a temperature between 1000 C. and 1200 C., and coolingit at a rate sufliciently slow to insure that substantially all of thecarbon is in the form of discrete particles of graphite so distributedas not to have any seriously detrimental effect on the magneticproperties of the sheet.

' WALTER CRAFTS.

